baker



July 21, 1959 Fig. I

w. E. BAKER 2,896,163

HOT WIRE DEVICE Filed Dec. 9, 1955 I l2 36 2s INVENTOR.

WILLIAM E. BY

ATTORNEYS will lengthen as its temperature increases.

United States Patent HOT WIRE DEVICE William E. Baker, Wellesley, Mass., assignor to Standard- Thomson Corporation, Waltham, Mass, a corporation of Delaware Application December 9, 1955, Serial No. 552,019 8 Claims. (Cl. 324-106) The present invention relates to hot wire devices, and more particularly to an improved construction thereof exhibiting marked properties of stability and precision of operation.

The principles of hot wire devices, so-called, are well understood. and have been the subject of intensive re search for a number of years. Essentially, these devices operate upon the principle that if current is passed through a length of wire having appreciable resistivity, it In this Way it is possible to provide a mechanical movement that corresponds to changes in the current passing through the wire. In one form which may be termed a discrete resolution device, the wire is caused to move one electrical contact in relation to another so as to make or break a circuit. In another form which may be termed an infinite resolution device, the changes in length of the wire are caused to move a pointer in relation to a dial, as in an ammeter or volt meter, or the like.

One of the difficulties with conventional hot wire devices resides in the fact that they operate upon very small changes in the length of the hot wire. Thus in any practical application, the available mechanical movement is small, and the error that may result from undesired movement of an end of the wire, as by slippage, is almost always appreciable in relation to the change of length within the normal range of operation.

The foregoing difficulty is accentuated by the fact that in a conventional hot wire device one end of the Wire must be supported in insulated relation to the other end because the Wire is connected with the electrical circuit at its ends. In consequence, the conventional construction is such that one end of the Wire is secured to a suitable metallic base or bracket, while the other end is supported by an insulator which is in turn secured to the bracket. It has been found that the dimensional instability of the insulation has produced undesired movement of the end of the wire due to humidity and other conditions. Also, in addition to such instability, there are problems of a considerable nature in attempting to form a sufliciently rigid bond between an end of the hot wire and the insulation, and of similarly rigidly securing the insulation to the support.

It is therefore an object of the present invention to eliminate the foregoing difficulties of conventional hot wire devices by providing a construction in which the insulation that must necessarily form a part of any hot, wire device is removed from the stress and creep sections thereof. In this way dimensional instability or insecure supporting of the insulation will have a negligible effect upon the overall performance of the device.

Another object is to provide a hot wire device for discrete or infinite resolution suitable for use as a thermostatic switch, as the actuating element in a meter operative over a continuous scale, or as the electromechanical transducer element in any device adapted to produce a mechanical displacement as a function of the value of an electric current. i

With the foregoing and other objects in view, the features of the invention include a structural arrangement in which both ends of the hot wire are firmly secured directly to rigid metallic elements, at the same electrical potential, the wire having a connection intermediate the ends leading to the source of electrical current. As a consequence of this arrangement, a further feature is that neither end of the wire is secured to insulation which, through mechanical instability, might cause an apparent change in the length of the wire. Since the ends are both firmly secured to rigid metallic parts, any length differential detected by the device is necessarily the desired result of the thermal effect of the applied current on the wire.

As a further feature, any forces applied to insulation that is employed to isolate the intermediate connection on the wire from the rigid metallic supporting parts are lower than would be the case for a conventional hot wire device having an insulated end connection, and in any case dimensional instability of the insulation can produce no undesired movement of the wire.

Other features of the invention reside in certain features of construction, modes of operation and arrangements of the parts that will be evident from the following description thereof, and from the appended drawings illustrating two embodiments thereof.

In the drawings,

Fig. 1 is a plan view of a hot wire switch embodying the invention;

Fig. 2 is a side elevation of the device shown in Fig. 1; and

Fig. 3 is a side elevation of a second embodiment of the invention incorporated in an ammeter.

Referring to Figs. 1 and 2, there is shown a hot Wire relay having a rigid metallic bracket or base 12, the base being electrically conductive and preferably but not necessarily constructed of stainless steel or some other noncorrosive metal.

An insulator 14 is secured to the underside of the base 12 by screws 16, and supports a fixed spring contact 18 and a movable spring contact 20, the contacts being also mutually insulated. The contacts 18 and 20 are adapted in any conventional manner for connection to an external circuit to be controlled by opening and closing of the contacts. Movement of the contact 20 is effected by an insulating push plate 22 formed of phenolic resin or other suitable plastic having a hole 24 to receive an end of the spring 20. The push plate 22 is projected through a rectangular hole 26 punched in the base 12.

A metallic spool 28, preferably of brass, is received through a hole in the push plate 22 and swaged securely thereto. The spool preferably has an annular groove 30 to facilitate connection of a flexible stranded wire 32 with the spool. Since this connection is not subjected to the tension on the Wire 34 any suitable mode of electrical connection may be employed, for example soldering.

A length of thermosensitive wire 34 is received through an axial hole in the spool 28, and bonded securely at one end by soldering or Welding to an ear 36 integral with and projecting upwardly from the base 12. At the opposite extremity of the wire 34, it is similarly welded or soldered at 38 to a rigid metallic adjustment member 40. The member 40 is secured by a rivet 42 to a flexible metallic leaf spring 44, the spring 44 being similarly riveted by a rivet 46 to a downwardly projecting end lip on the base 12.

A bias spring 48 is secured to the rivet 42 at one end, and to a downwardly projecting car 50 integral with the base 12 at the other end. The spring 48 is under tension, urging the member 40 to rotate clockwise, as viewed in Fig. 2, about a pivot ridge 52 formed in the member 40. The torque exerted by the wire 34 on the member 3 40 is less than that exerted by the spring 48. Clockwise rotation of the member 40 is therefore limited at all times by a shouldered adjusting screw 54 threaded into the base 12. Thus rotation of the screw 54 effects a change in the position of the right-hand end of the wire 34 as viewed in Fig. 2.

It will be observed that the position of the push plate 22 is that position in which the forces exerted thereon by the movable contact spring 20 and the wire 34 are in equilibrium. Under these conditions there is a negligibly small tension on the wire 34 and therefore no appreciable strain arising from this cause.

' The flexible wire 32 is connected by a screw 56 to an insulator 58, the insulator being secured by a rivet 60 to the base 12. v r

The electrical circuit for sending current through the wire 34 is connected between the screw 56 and a screw 62 threaded directly into the base 12. Thus the current passes from the source to the connection 56, through the wire 32 to the center portion of the wire 34, through the halves of the wire 34 in parallel, thence through the base 12 to the screw 62, and back tothe source.

If it is desired to provide a circuit having a continuous fused metallic path, the spool 28 may be welded or soldered to the wire 34 as indicated at 64. In this case any possible variation of impedance due to a change in the pressure of the spool 28 upon the Wire 34 is eliminated.

It will be observed that the insulator 56, which is analogous to the insulator found at one end of a conventional hot wire device, is under negligible tension, and any dimensional instability therein due to ambient conditions can have no eflect upon the length or position of the wire 34.

An increase in the current flowing through the wire 34 causes it to heat up due to its resistivity, and to lengthen by a differential proportional to its length, change in temperature and the coeflicient of linear thermal expansion of the metal. This change in length allows the push plate 22-to move upwardly as viewed in Fig. 2 causing the contact 20 to move toward the contact 18. The control point of the device, that is, the current value at which the contacts 18 and 20 close, is controlled by rotation of the screw 54. If the current passing through the wire 34 is reduced below the value corresponding to the control point, the contact 20 falls back from the contact 18 and the external circuit connected therewith is opened.

It will be obvious that break contacts or transfer contacts may be actuated in a similar manner if desired, simply by substitution of such contacts for those illustrated in the drawing.

From the foregoing it will be appreciated that the rate of production of heat in the wire 34 is in any case proportional to the square of the current flowing through it. Also, at any given value of current the wire reaches and tends to maintain a corresponding temperature at which the rate of heat dissipation through conduction, convection and radiation from the wire equals the rate of production of heat in it.

A second embodiment of the invention is illustrated in Fig. 3. In this case the output of the device takes the form of a pointer positioned with reference to a continuous scale 64. The structure of the electromechanical transducer component is substantially the same as that illustrated in Figs. 1 and 2. A base 66 supports a hot wire 68 between a fixed end 70 and an adjustable end 72, the end 72 being secured to a member 74 urged in a clockwise direction about a pivot 76 by a bias spring 78 under tension. A shouldered adjustment screw 80 limits the position of the member 74.

The wire 68 passes through a brass spool 82 swaged onto a suitable insulating push lever 84, the lever 84 being preferably constructed of phenolic resin or other suitable plastic substance. A stranded flexible wire 86 is 4 secured to the spool 82 and to an insulator 88. A screw 90 is threaded into a metallic post 92, the post 92 being in turn secured to the base 66. The external circuit for sending current to the wire 68 is connected between a screw 94 threaded into the insulator 88 and the screw 90. At its lower end the push lever 84 is pivotally secured to a lever 96, the lever 96 being rotatable about a fixed axis 98. At its opposite end the lever '96 has a circular toothed segment 100 in mesh with a pinion 102 secured to a rotatable shaft 104. A pointer 106 associated with the scale 64 is also secured to the shaft 104. A bias spring 108 is fixed at one end to a post 110 and at the other endto the rotatable shaft 104. For a zero adjustment, the screw 80 is turned until the lever 106 moves to the 0 position on the dial, in which position the reaction of the spring 108 produces a torque on the lever 96 equal to and opposite the torque exerted through the push lever 84 by the wire 68.

device is of a continuously varying nature rather than a discrete nature, as in the case of Figs. 1 and 2.

It will be understood that, while the invention has been described with reference to preferred embodiments thereof, the illustrated structures may be modified as to design and details of structure in accordance with skills presently knovm to the art without departing from the spirit or scope of the invention.

Having thus described the invention, I claim:

1. An electro-mechanical transducer having the combination of a metal support, a continuous metallic strand secured at both ends to the support, deflecting means bearing upon the strand intermediate its ends, the strand being suspended between said ends without support except by said deflecting means, resilient means tocause the deflecting means to tend to deflect the strand transversely from the line between its secured ends, whereby the position of the deflecting means is a function of said deflection, control means associated with the deflecting means and operative as a function of its position, a flexible conductor connected with the strand intermediate its ends, and connections for passing current through said conductor and through two portions of the strand in parallel to the support member.

2. An electro-mechanical transducer having the combination of a metal support, a continuous metallic strand secured at both ends to the support, deflecting means bearing upon a single point of the strand, the strand being suspended between said ends without support except at said point, resilient means to cause the deflecting means to tend to deflect the strand transversely from the line 7 between its secured ends, whereby the position of the deflecting means is a function of said deflection, control means associated with the deflecting means and operative as a function of its position, a flexible conductor connected with the Strand intermediate its ends, and connections for passing current through said conductor and through two portions of the strand in parallel to the support member. A

3. An electro-mechanical transducer having the com: bination of a metal support, a continuous metallic strand secured at both ends to the support, deflecting means bearing upon a single point of the strand intermediate its ends, the strand being suspended between said ends without support except at said point, resilient means to cause the deflecting means to tend to deflect the strand transversely from the line between its secured ends, whereby the position of the deflecting means is a function of said deflection, control means associated with the deflect.-

ing means and operative as a function of its position, a flexible conductor connected with the strand substantially at its center, and connections for passing current through said conductor and through two substantially equal portions of the strand in parallel to the support member.

4. An electro-mechanical transducer having the combination of a metal support, a continuous metallic strand secured at both ends to the support, deflecting means hearing upon a single point of the strand substantially at its center, the strand being suspended between said ends without support except at said point, resilient means to cause the deflecting means to tend to deflect the strand transversely from the line between its secured ends, whereby the position of the deflecting means is a function of said deflection, control means associated with the deflecting means and operative as a function of its position, a flexible conductor connected with the strand through said deflecting means, and connections for passing current through said conductor and through two substantially equal portions of the strand in parallel to the support member.

5. An electro-mechanical transducer having the combination of a metal support, a continuous metallic strand secured at both ends to the support, deflecting means bearing upon a single point of the strand intermediate at its ends, the strand being suspended between said ends without support except at said point, resilient means to cause the deflecting means to tend to deflect the strand transversely from the line between its secured ends, whereby the position of the deflecting means is a function of said deflection, control means associated with the deflecting means and operative as a function of its position, an insulator fixed to the support, a flexible conductor secured at one end to the insulator and at the other end to the strand intermediate its ends, said conductor applying no appreciable force to the strand, and connections for passing current through said conductor and through two portions of the strand in parallel to the support member.

6. An electro-mechanical transducer having the combination of a metal support, a continuous metallic strand secured at both ends to the support, the support including means for adjusting the distance between said ends, defleeting means bearing upon a single point of the strand intermediate its ends, the strand being suspended between said ends without support except at said point, resilient means to cause the deflecting means to tend to deflect the strand transversely from the line between its secured ends, whereby the position of the deflecting means is a function of said deflection, control means associated with the deflecting means and operative as a function of its position, a flexible conductor connected with the strand intermediate its ends, and connections for passing current through said conductor and through two portions of the strand in parallel to the support member.

7. An electro-mechanical transducer having the combination of a metal support, a continuous metallic strand secured at both ends to the support, a pair of spring contacts secured to the support, deflecting means bearing upon a single point of the strand intermediate its ends and resiliently engaged by one of said contacts to cause it to tend to deflect the strand transversely from the line between its secured ends, whereby the contacts are opened or closed as a function of said deflection, the strand being suspended between said ends without support except at said point, a flexible conductor connected with the strand intermediate its ends, and connections for passing current through said conductor and through two portions of the strand in parallel to the support member.

8. An electro-mechanical transducer having the combination of a metal support, a continuous metallic strand secured at both ends to the support, deflecting means bearing upon a single point of the strand intermediate its ends, a displaceable indicator member, a mechanical connection joining said displaceable member with said deflecting means, resilient means to cause the deflecting means to tend to deflect the strand transversely from the line between its secured ends, whereby the position of the deflecting means is a function of said deflection, the strand being suspended between said ends without support except at said point, a flexible conductor connected with the strand intermediate its ends, and connections for passing current through said conductor and through two portions of the strand in parallel to the support member.

References Cited in the file of this patent UNITED STATES PATENTS 926,536 Babcock June 29, 1909 992,427 Jandus May 16, 1911 1,288,947 McIver Dec. 24, 1918 

